The storage and transport of materials that have a tendency to stick to each other and to other materials requires that so-called separator films are inserted between neighboring layers of the sticking material and that the sticky material is packed in such film material. The sticking material may, for example, comprise a plurality of so-called prepreg layers which include fiber reinforced synthetic materials in which the fibers are embedded in a resin matrix material that has not yet been cured. Once these prepregs are ready for use, for example, in the formation of an aircraft wing or the like, the separation film must be removed completely. If the separation film is not removed completely, then the resulting product does not meet the required strength characteristics and must be rejected. The complete removal of the separation film poses a problem because due to tearing of the film, portions of the film may remain on the surface of individual prepregs and such portions are undesirable because, as mentioned, they reduce the material strength of the final product.
The problem of removing the entire separation film is aggravated by the fact that the films conventionally employed are very hard to see against the background of the sticking material on which the film has been previously applied. This problem is especially aggravated when the prepregs are carbon fiber reinforced because in that case the material is black and the film is practically not visible to the naked eye against the black background.
The foregoing considerations also apply to so-called support plies or substrates which may be of the same material as the separation films, but somewhat stronger, especially stiffer, so as to provide some support for the prepreg layer prior to the curing of the matrix material so that such a prepreg material may be properly stored and transported prior to curing. Storage and transport of these materials usually takes place in the form of stacks or in the form of rolled-up bales. The separation films are primarily used for the formation of rolled-up bales, while the support plies are used for the formation of stacks of prepregs. Films and plies may also be used in combination.
The foregoing considerations also apply to packing film material that is used to envelope preformed fiber reinforced structural components, the matrix material of which has not yet been cured.
A plurality of prepregs forming a stack may be enclosed by the present material to form a package in which the individual prepreg is separated from its neighbors by separator film layers. In this type of use as a packing material it is also necessary that prior to a curing operation the protective packing film must be removed completely to obtain the intended characteristics of the finished product.
Packing material of this type is made either out of paper or synthetic materials and may be in film form. In order to improve the separating ability of the film material from the matrix material, it is known to treat the surface of the film material with a separating agent such as paraffin or the like. Conventional film material of this type is substantially transparent so that it becomes difficult to see whether the film material has been removed from the prepreg, especially whether it has been completely removed from all surface portions of the prepreg layers to be separated from one another or from the surface of a structural component. The poor visibility due to film transparency, especially when black prepregs are involved, makes it very difficult and hence time consuming to completely remove the conventional film material. As a result, it has been a problem heretofore that these film materials or portions thereof have been overlooked during inspection so that film or film portions become part of the structure into which the prepregs are integrated. Due to the separating effect of the film material the structure becomes a reject.
The need for the complete removal of the packing film has been recognized heretofore. However, the conventional practice for such removal is to provide lists of all individual film sections and these lists must be checked by the operator against the removed film material, whereby an incompletely checkable list indicates that respective portions of film materials have gone undetected. Thus, additional visual inspections were necessary prior to the invention, even after the films have been removed. Such subsequent inspections are also very difficult due to the lack of suitable contrast conditions between the transparent film and the packaged or separated material. Another disadvantage of the conventional approach to solving this problem is the fact that visual inspections by a human operator are not very suitable for integration into a substantially automated manufacturing operation.
The foregoing problem is further aggravated when prepreg layers in the form of precut blanks have been separated from the separator film and layered to form the structural component. If, in such a case, pieces of separator film remain inside the layered structure, such separator film pieces cannot be detected at all by conventional means.